The overall goal of this proposal is to determine the mechanism(s) of synergistic action of the natural source compounds, known to inhibit one or more stages of skin carcinogenesis, i.e., initiation and promotion/progression. Our hypothesis is that concurrent topical and systemic (i.e., dietary) treatment with selected natural source inhibitors of different stages of skin carcinogenesis result in synergistic effects leading to more efficient prevention of skin cancer. The inhibitors to be tested include ellagic acid, proanthocyanidin B-2-gallate, N-acetylcysteine, calcium D-glucarate, lycopene, ursolic acid from rosemary extract, and resveratrol. We propose to initially utilize a number of very predictive short-term in vitro and in vivo tests in order to identify the mechanism(s) and to differentiate the potencies of selected inhibitors at various concentrations under standard conditions. The most effective compounds will then be studied in long-term tumor experiments utilizing a 7,12-dimethylbenz[a]anthracene (DMBA)-initiated, 12-O-tetradecanoylphorbol-13-acetate (TPA)- promoted multistage carcinogenesis model in SENCAR mice and an ultraviolet light (UV)-initiated, TPA-promoted multistage carcinogenesis model in SKH-1 mice. Finally, combinations of the best anti-initiating and anti-tumor promoting agents will be tested in the DMBA-initiated, TPA-promoted skin carcinogenesis model in SENCAR mice as well as in the UV-initiated, TPA-promoted skin carcinogenesis model in SKH-1 mice. The following Specific Aims will be studied: (i) identification, using various very predictive short-term in vitro and in vivo tests, including the DMBA-induced inflammatory-hyperplasia assay in SENCAR mice and the UV-induced erythema-hyperplasia assay in SKH-1 mice, the most promising mechanisms of anti-initiation and anti-tumor promotion as well as the most effective natural source inhibitors and their delivery means, i.e., topical vs. systemic (dietary); (ii) analysis of the long-term anti-initiation and antitumor promoting effects of the selected, promising natural source inhibitors using the DMBA-initiated, TPA-promoted multistage skin carcinogenesis model in SENCAR mice to better understand their mechanism(s) of action and predict their performance in the following combination studies; (iii) studying the long-term inhibitory effects of the best anti-initiators and anti-tumor promoters identified in Aims 1 and 2, on UV-initiated, TPA-promoted multistage skin carcinogenesis in SKH-1 mice, in order to establish a bridge between the chemically induced and UV-induced mouse skin carcinogenesis data bases; (iv) examining how various combinations of mechanistically different anti-initiators and antitumor promoting agents and also how various combinations of their delivery means, i.e., topical vs. systemic (dietary), inhibit DMBA-initiated, TPA-promoted skin carcinogenesis in SENCAR mice as well as UV-initiated, TPA-promoted skin carcinogenesis in SKH-1 mice, with the view of discovering the most pronounced synergistic effects of the selected natural source inhibitors.